The Use of Stable Isotopes to Evaluate the Uptake Efficiency of Applied Nitrogen Fertilizer in Southern Loblolly Pine Plantations.

Fertilizer studies in forest plantation systems across the Southern United States indicate 10-25% of nitrogen (N) applied during fertilization is incorporated into crop trees.  The remainder of the N is retained by different components of the ecosystem, volatilized or is leached.  A comprehensive understanding of the movement of N in these systems is critical due to financial costs associated with fertilization, and an environmental awareness of the potential negative impacts from improper fertilizer application.  Our primary research objective is to improve the fundamental understanding of N dynamics in forested systems by 1) determining the fate of applied fertilizer N in mid-rotation loblolly pine (Pinus taeda) stands (8-25 years) across the entire plantation range in the Southern United States by using ¹⁵N enriched fertilizers (0.5 atom %;~370 ‰), and 2) compare N dynamics following fertilization with conventional (urea) and enhanced efficiency (polymer coated urea, urea+NBPT, monoammonium phosphate coated urea) fertilizers enriched with ¹⁵N.  Five 100 m² circular plots were installed at each of the 20 study sites across the entire range of loblolly pine plantations from 2011-2012 in a wide range of soil and stand conditions.  During the late winter months, 224 kg ha^-1 of total N was applied to each treatment plot.  The rate of N uptake from different fertilizers is being evaluated by periodic foliage sampling in 2011.  Total N uptake in the growing season is being determined by destructive biomass sampling of a tree and separation into individual components (foliage, fine branches, coarse branches, bole) to determine the mass and concentration in each component.  Additional components of the ecosystem are being analyzed to determine the fate of applied N (understory vegetation, forest floor, mineral soil, roots).  N volatilized following fertilization was measured using a static chamber method and a microcosm experiment.